I don't think you can translate it "friendly" unless you do so as a substantive adjective, where the noun is impied but not actually present. But that woud translate as: "Your virtue makes me a friendy [noun] to you", which isn't particuarly fluid. Apart from that, there's no noun for "amicum" to modify as an adj, so all in all, I think it's best to translate it as a noun in its own right.

So far as the specific example is concerned, there was nothing said at that point to clarify the intention of the phrase, but chapter 14 does have a similar sentence in one of the examples wherein the author does briefly address how to handle cases such as this. SA 1 reads (page 94): Et Deus aquas maria in principio appellavit. Both 'aqua' and 'mare,' in this example, are in the accusative plural, and the author goes on to explain that, with certain verbs not limited to but including 'appello' and 'facio,' two accusatives can be included where one is translated in the normal sense and the other as a 'predicate accusative' or 'objective complement.' In the case of the sentence above, the most straightforward translation would treat 'maria' as the adjective to read: And God called the sea waters in the beginning. As for your own example, I guess I like to play fast and loose with the translations, because I would have been content in telling you to translate that as, "Your virtue makes me your friend." So long as the sentence conveys the same meaning, and treats the original passage's wording with its due respect, I can't imagine anyone should reproach you for not being as literal as possible. The absence of definite and indefinite articles and the regular omission of possessive pronouns makes Latin, at least to me, a bit more subtle and flexible in its translation. But, as someone already said, "Your virtue makes me friendly to you" would be the most literal translation. Sorry for the belated response, but I know I felt better when I found the author addressing the issue in question only 3 chapters later, and despite the fact the first example left little ambiguity. Bonam fortunam, amice!